The present invention concerns a watch movement, in particular a watch movement with a microgenerator. The present invention also concerns a method for testing such watch movements.
Watch movements with a microgenerator have been described notably in the patent documents CH597636 (Ebauches SA) and EP0851322 (Ronda SA). In such a watch movement, the balance known from mechanical watch movements is replaced by a generator 10-22 (FIG. 2) and an electronic regulating circuit 81 with a quartz oscillator 85. The generator is driven by a spring (not represented) over a part of the gear train 50, 60, 70 (FIG. 1). The generator feeds the electronics that in turn regulate the rotational speed of the generator and thus the running of the watch movement. Such watch movements therefore combine the advantages of a mechanical clock with the precision of a quartz watch.
The forces, moments and rotational speeds that are effective in such a watch movement correspond roughly to those in a mechanical clock. Thus, it is to be expected that the wear would be more or less the same.
The present invention is based on the observation that is surprisingly not the case. In such watches, strong signs of wear appear after a short time.
It has been observed, for example, that the oil in the jewel bearings deteriorates within a short time period. Furthermore, strong signs of wear have been noticed at the addendums of the teeth.
Wear has also been noticed in places where the teeth never touch, for example precisely at the teeth cusps. A lot of abrasion has also been found in the oil on the jewel bearings. The faster the wheel rotates, the stronger the destruction of the oil at the bearings of the corresponding wheel.
It is one aim of the invention to build a watch movement with a microgenerator that does not show these problems.
It is another aim of the invention to construct a watch movement with a microgenerator that is at least as durable as a conventional mechanical watch movement.
It is another aim of the invention to build a cheap and, in addition, reliable watch movement that is controlled with a generator and in which these wear problems do not occur.
According to the invention, these aims are achieved by means of a microgenerator having the characteristics of the characterizing part of claim 1, preferred embodiments being further indicated in the dependent claims.
These aims are achieved specifically by understanding the phenomenon that causes the rapid wear.
The aforementioned problem was solved in particular by discovering a totally unexpected effect in such watch movements and by inventing solutions to prevent this effect.
The essential difference between a mechanical watch movement and a generator watch movement lies in the electric grounding of the components. In a conventional mechanical clock, the balance is electrically grounded directly over the spring coil. In a watch movement with a microgenerator, the rotor 10 of the generator should also be grounded electrically over the train 50, 51, 60, 61, 70, 71. But, as measurements have shown, this is surprisingly not the case: the rotor is insulated from the plate of the watch movement.
The explanation found in the framework of this invention for this surprising fact is the following: as the driving torque at the generator is very small and the magnets 12 of the rotor stray fields, the axis 50 of the wheel 51 driving the rotor may not be magnetic. Otherwise, the rotor receives a positioning torque substantially greater than the driving torque available to the generator, which causes the generator to stop. To prevent this, the axis in question is made of copper-beryllium (CuBe). This solution has already been described in the above-mentioned application EP0851322. Copper-beryllium however has the tendency to develop layers of oxide. If this oxide layer is thick enough and the surface pressure in the gearing is small, the rotor 10 as well as the wheel 51 and the pinion 50 (Inter2) driving the rotor can be electrically insulated from the rest of the watch movement.
On the other hand, if the generator 10, the pinion 50 and the wheel 51 are electrically insulated from the other parts of the watch movement, they can be charged electrically through frictional electricity and/or through the rotor""s stray fields that induce a voltage in the wheel 50-51. As soon as the voltage has reached a certain value, there can be a discharge of sparks, as described below, which can lead to a more rapid wear of the gear train and a rapid deterioration of the lubrication.
The insulated wheels and the rotor can be charged especially through frictional electricity. If two surfaces are in contact and then separate, electrons will be torn from one of the surfaces, with the result that one body has a negative and the other a positive charge. If the bodies are not electrically insulated from one another, the charges will simply be exchanged again at the next contact.
If on the other hand the bodies are insulated from each other, for example by a layer of oxide, these charges cannot be exchanged, so that the bodies will be charged.
Charges with the same polarization repel mutually, leading to the charges being at maximum distance from each other. Because the separation of the charge occurs on the little pinion, the charges have the possibility of spreading onto the big wheel, so that the pinion is no longer charged and can be recharged at the next separation. The well-known Van den Graaf generator works according to this principle. In this manner, a charging pump results that deposits the charges on the rotor 10. If it is assumed that the engagement between the rotor 10 and wheel 51 yields about 7,000,000 meshings and between the pinion 50 and the wheel 61 about 1,000,000 meshings per day, it is evident that in this way considerable voltages build up.
As soon as the voltage developed in this fashion is bigger than the breakdown voltage of the insulation layer, there is an exchange of charge. Depending on the voltage, a spark discharge may occur.
If then the rotor 10 is electrically insulated from the rest of the watch movement, as demonstrated by measurements of the electric resistance between the plate 30 and the rotor 10, it is charged, either through air friction, through charge separation as described further above or through the voltage induced in the wheel 50-51 by the magnetic stray fields of the rotor 10.
If the voltage built up through friction electricity and/or through the rotor""s stray fields is too big for the electric insulation, there are discharges. This can be spark discharges in the meshing or there can be other discharges, for example directly between the rotor 10 and the plate 30. These discharges cause the following damage in the watch movement:
There is a lot of abrasion at the teeth cusps of the wheel 61 (Inter 1), the teeth cusps are heavily damaged, though these teeth cusps are never in contact with teeth of the other wheel.
On the pinion 50 (Inter 2), quite a thick layer of oxide develops. Here, too, the teeth cusps are partially destroyed. Furthermore, there are traces of abrasion on the teeth flanks.
The oil of Inter (60-61), Inter 2 (50-51) and generator 10 is deteriorated, on the one hand by the formation of ozone, on the other hand by the high electric voltage and the spark discharge.
In the bearings 41, there are traces of abrasion and the oil is full of small particles.
The teeth of the wheel are soiled with abrasion particles.
The pegs are heavily worn out because of the particles in the oil.
The different chemical substances in the oil attack the pegs chemically.
The electronics 81 may possibly be disturbed by the discharges.
These problems occur only after a certain time, but if they do, the watch movement stops after a short time. Once there are spark discharges, the layer of oxide grows, as does the tendency to charge the wheels through frictional electricity, and the damages continue with ever growing intensity. After a short time, the friction caused by the deteriorated oil and the dirt in the jewel bearings is so great, that the driving force available at the generator is smaller that the needed driving force, so that the regulation does not function any more.
These experiments according to the invention were carried out under a scanning electron microscope in order to check whether the wheels in the train can be charged. In this process, an electron beam is focused on the rotor 10. If the rotor can be charged, it means that it is not grounded over the train 50, 51, 60, 61, 70, 71 of the plate 30, i.e. it is not insulated from the plate.
Spark discharges could be observed in the scanning electron microscope, which demonstrates that the rotor 10 is electrically insulated. The damage visible on the wheels in the train looks very similar to the damage that happens in watches after a wear test of several months.
In order to solve the problem of the watch movements with a microgenerator according to the state of the art, the gearing is grounded, in a first embodiment of the invention. Thus, an electric charging of the rotor and of the gearing is avoided. It is for example possible to ground the gearing over the meshing or over the axes, for example in the bearings or by means of brush contacts on the axes.
In a second embodiment of the invention, which may be combined with the first embodiment, charge separation is prevented. The occurrence of charge separation can for example be avoided by using materials that have approximately the same electrochemical potential and/or the same dielectric constant. If the materials that are in contact with each other possess approximately the same surface characteristics, the tendency of electrons being torn away when there is a separation of the materials is not very high. Therefore, materials or surfaces with good tribological characteristics and a hardness greater than 200DH can for example be used.
In a third embodiment of the invention, which may be combined with the first and/or second embodiment, oil that is resistant to ozone is used. This allows for the lubrication to be kept intact, if within the watch movement ozone is regularly produced by spark discharges.
In a fourth embodiment of the invention, which may be combined with the first and/or second and/or third embodiment, jewel bearings are used that protect the oil as much as possible against oxidation. This is achieved by keeping the jewel bearings as closed as possible, on the one hand in order to keep the oil in the bearings by capillary effect and, on the other, in order that the oil is thus not exposed to oxygen and the possible ozone it contains.